My scout troop has about 10 ropes in storage. They were bought between January of 2000 and October of 2001. They live in a closed plastic tote 365/24/7. At most, they have been used 5 times for top roping. Due to age though, they all have to be retired.
It breaks the heart though to see an unused (seriously, never even uncoiled) 10.2 mm x 60 meter Sterling dynamic lined up for destruction. If it's gotta go, it's gotta go, but it sure does hurt.